The present invention relates to a method and an apparatus for detecting defects in a recording medium, and in particular to a method and an apparatus for detecting an extremely minute defect on a recording medium for high density recording such as a magnetic disk.
In recent years, magnetic disk units have been required to have increased recording capacities. Therefore, the recording capacity per head disk assembly (hereafter referred to as HDA) has now reached 2.5 gigabytes (G bytes). It is expected that the surface recording density will increase by approximately ten times the present density in the future. A problem incurred when the surface recording density is increased is the occurrence a defect on the recording medium. As the surface recording density (the number of bits recorded per unit area) is increased, the area of the recording medium occupied by one bit is correspondingly decreased. Accordingly, minute defects which posed no problems for the surface recording density in the past exert bad influence upon the information (data) writing and reading operation.
This will now be described by taking the improvement of the surface recording density over the past ten years as an example. Whereas the surface recording density of the magnetic disk unit was 1 megabits/(inch).sup.2 approximately 15 years ago, it has been raised to 25.6 Mbits/(inch).sup.2 in recent years. That is to say, the surface recording density of the magnetic disk unit has been increased by 25.6 times. Assuming that a defect within an area in which one bit data is recorded of the disk is not changed, for example, the ratio of the defect within an area in which one bit data is recoded of a disk unit in recent years amounts to 25.6 times that of a disk unit in the past. Accordingly, a defect which did not cause any problem in the past has posed a serious problem in recent years.
Detection of minute defects on the disk is an important subject for maintaining the reliability of the magnetic disk unit. In a known method for detecting defects, an amplitude signal is recorded on the recording medium beforehand and the variation in the amplitude value of the amplitude signal is measured to detect defects.
However, the amplitude variation is caused by not only defects but also other various factors. In the conventional method for detecting amplitude variation, therefore, a number of changes which have no relation to actual readout reliability are extracted. Thus the conventional method is not suitable to detection of minute defects, resulting in a problem.
A method obviating this problem of the amplitude variation detection method is disclosed in JP-A-No. 59-217230. In this method, minute defects are detected by measuring the variation in phase direction of the peak point on the amplitude of the reproduced signal waveform. If only the phase variation of the peak value is detected as described in the aforementioned publication, however, all of a plurality of reproduced signal frequencies must be measured and inspected. This results in a problem of inefficient disk inspection.